1. Field of the Invention
The present invention relates to a position indicator for use with a digitizer adapted to input position information, a variable capacitor suitably used in the position indicator, a position input device including the position indicator and a digitizer, and a computer system using the position input device, particularly in which the structure of the variable capacitor used in a pen pressure detecting section of the position indicator is simplified and therefore the radial dimension of the position indicator is reduced.
2. Description of the Related Art
A variable capacitor is conventionally used in a pen pressure detecting section of a position indicator for an electromagnetic induction type position input device (see Japanese Patent No. 3150685, incorporated by reference herein, for example). Examples of such a variable capacitor include the one in which an electrode on one surface of the variable capacitor is divided into two parts so that signals may be easily accessed from only one surface, as opposed to from both surfaces, of the variable capacitor (see, for example, Japanese Unexamined Patent Application Publication No. 2001-319831, corresponding to U.S. Pat. No. 6,853,369, incorporated by reference herein). However, both the variable capacitor disclosed in Japanese Patent No. 3150685 and the variable capacitor disclosed in Japanese Unexamined Patent Application Publication No. 2001-319831 require a large number of components.
FIGS. 10A and 10B show a concrete configuration of a variable capacitor disclosed in Japanese Patent No. 3150685. As shown in FIGS. 10A and 10B, the variable capacitor includes a dielectric 201, a first electrode 202, a second electrode 203, a ring-shaped spacer 204, an elastic body 205, two terminals 206 and 207, and a rod 210. The dielectric 201 is made of a hard material and is substantially formed in a disc shape having two surfaces 201a and 201b extending in parallel with each other. Hereinafter the surface 201a is referred to as “one surface 201a”, and the surface 201b is referred to as “the other surface 201b”. For example, the dielectric 201 is made of a ceramic material having a thickness of 2 mm, a diameter of 4.6 mm, and a relative dielectric constant (or relative permittivity) of 7000. The first electrode 202 is provided on the one surface 201a of the dielectric 201. The other surface 201b of the dielectric 201 is smoothly polished to a surface roughness (Ra) of 0.1 μm or less.
The first electrode 202 is made of a substantially disc-shaped silver plate having a thickness of 0.2 mm and a diameter of 4.0 mm. Further, the first electrode 202 is sintered onto the one surface 201a of the dielectric 201. The second electrode 203 is a flexible insulating film, for example. The second electrode 203 is formed, for example, by vapor-depositing a nichrome to a thickness of 1000 angstrom on a polyimide film having a thickness of 75 μm. The second electrode 203 includes a disc-shaped electrode portion having a diameter of 4.6 mm, and a terminal portion extending from the electrode portion into a tongue shape (the left-hand side portion of the second electrode 203 as shown in FIGS. 10A and 10B).
The spacer 204 is made of a polyimide film having a thickness of 40 μm and a relative dielectric constant of 3.5. The spacer 204 includes a ring-shaped main body having an outer diameter of 4.6 mm and an inner diameter of 3.3 mm, and an engaging portion extending from the main body into a tongue shape (the right-hand side portion of the spacer 204 as shown in FIGS. 10A and 10B). The elastic body 205 is made of, for example, a silicon rubber having a thickness of 0.35 mm. The elastic body 205 includes a disc-shaped main body having a diameter of 4.6 mm and two engaging portions. The two engaging portions respectively extend from two places opposite to each other in the radial direction of the main body into two tongue shapes (the right-hand and left-hand side portions of the elastic body 205 as shown in FIGS. 10A and 10B).
The terminals 206 and 207 respectively include disc-shaped electrode portions 206a and 207a and cylindrical lead portions 206b and 207b. The lead portions 206b and 207b respectively extend from the center of one surface of the electrode portions 206a and 207a in a direction perpendicular to the plate surface of the electrode portions 206a and 207a. The lead portions 206b and 207b are formed by plating nickel and gold on the surface of brass. When pen pressure is applied, the other surfaces of the electrode portions 206a and 207a are respectively brought into electrical connection with the first electrode 202 and the second electrode 203.
As shown in FIG. 10A, in a state where there is no pressure or displacement applied to the rod 210 (namely, in a state where the variable capacitor is in the initial state), the other surface 201b of the dielectric 201 and the second electrode 203 are spaced apart from each other by the spacer 204 by a distance equal to the thickness of the spacer 204, except for the peripheral portion. As a result, an air layer 208 is formed between the dielectric 201 and the second electrode 203. The capacitance value (the initial capacitance) between the terminal 206 and the terminal 207 is substantially a combined capacitance obtained by series-connecting the capacitance contributed by the dielectric 201 with capacitance contributed by the air layer 208 having a relative dielectric constant of 1.0, and therefore the combined capacitance is very small.
However, if a pressure or a displacement is applied to the rod 210, such a pressure or displacement will be applied to the second electrode 203 through the elastic body 205. As a result, the second electrode 203 will be bent toward the other surface 201b of the dielectric 201. Consequently, the thickness of the air layer 208 will become smaller than the thickness of the spacer 204. Since the capacitance contributed by the air layer 208 increases in inverse proportion to the thickness of the air layer 208, if the thickness of the air layer 208 decreases, the capacitance contributed by the air layer 208 will increase, and therefore the capacitance between the terminal 206 and the terminal 207 will increase.
After that, as shown in FIG. 10B, if the pressure or displacement applied to rod 210 increases to an extent such that the second electrode 203 is brought into contact with the other surface 201b of the dielectric 201, the capacitance in the electrode-dielectric contact area will become exactly the capacitance contributed by the dielectric 201 only. Thus, the capacitance value between the terminal 206 and the terminal 207 increases substantially in proportion to the size of the contact area. In such a manner, the capacitance value of the aforesaid variable capacitor changes largely in response to a pressure or a very small displacement applied to one end of the rod 210.